1. Field of the Invention
The present invention relates to a rubber composition for cleaning metal molds and a method for cleaning metal molds by using the rubber composition, and more particularly, to a rubber composition for cleaning metal molds the surfaces of which are soiled by repeated vulcanization and a method for cleaning the metal molds.
2. Description of the Related Art
In the rubber molding industry, it has been known for a long time that metal molds for vulcanization are soiled. Dirt of metal mold is accelerated in a vulcanization step as the frequency in use increases and adversely affects quality, appearance, precision of size and the like, and further the dirt makes releasability of molded rubber from metal molds poor and shortens the life of metal molds.
Therefore, there have been proposed various counter-measures to prevent dirt of metal molds. For example, Japanese Patent Publication No. 44980/1984 discloses a method comprising adding organic phosphoric acid esters or salts of cyclopentadiene derivative carbonxylic acids to outer surface liquids, and Japanese Patent Application Laid-open No. 113518/1987 discloses a method comprising treating the surface of a metal mold with a compound containing silicon and isocyanate group.
As methods for decreasing dirt of metal mold by incorporating particular substances in rubber compositions to be molded, there have been proposed to incorporate in rubber compositions cyclized polybutadienes (Japanese Patent Application Laid-open No. 63144/1979), organic phosphoric acid esters (Japanese Patent Publication No. 53812/1982), mercaptobenzo-1,3-diazole salts (Japanese Patent Application Laid-open No. 107943/1979), metal salts of cyclopentadiene derivative carboxylic acids (Japanese Patent Application Laid-open No. 126251/1979), 2-mercapto-benzimidazole or zinc salt thereof (Japanese Patent Publication No. 20585/1986), or particular silica (hydrated silicic acid, Japanese Patent Application Laid-open No. 238143/1988).
These methods can not completely prevent the dirt though these can defer the occurrence of dirt. Therefore, periodical cleaning of metal molds such as a shot-blast method, an acid or alkali cleaning and the like are practiced, but these cleaning methods damage the metal molds, accelerate the dirtying speed after the cleaning, and cause corrosion of metal molds, and moreover, it is necessary to remove the metal molds from curing presses, and in case of large scale rubber products, the labors are not negligible and therefore, the productivity is lowered.
As methods for cleaning metal molds without removing the metal molds from a curing press, Japanese Patent Application Laid-open No. 122942/1983 discloses a method where a rubber composition containing an aminoalcohol is used as a cleaning rubber, and Japanese Patent Application Laid-open No. 180600/1983 discloses a method where an aminoalcohol or a solution of an aminoalcohol in ethanol is applied to the dirty surface of a metal mold and then a non-vulcanized rubber composition is vulcanized therein according to a conventional method to clean the mold.
Other than the above-mentioned cleaning methods, various rubber compositions for metal mold cleaning are commercially available and those exhibit cleaning effect to some extent, but the effect is not sufficient. Moreover, most of such cleaning rubber compositions contains a large amount of easily decomposable amine type chemicals so that the compositions have ammoniac smell even at room temperature, and after heating, much more amines are generated and thereby metal molds are corroded or working environmental problems occur.